Work stoppage because of scratch disk error. The torture continues! Please help!

Question

Hi, everyone:
I would greatly appreciate help.
In December, I began receiving the error message Could not initialize Photoshop because the scratch disks are full. I deleted all the large files from my hard drive, and the problem went away.
Now, I am working on a large Photoshop file (i.e., 585,155kb). I am working at 300 ppi for print reproduction, using multiple layers. I am opening each new image to be added to the master document in a separate Photoshop window. I set the resolution of each at 300 ppi, then crop and transfer to its new layer in the master file (using drag and drop). After I crop and transfer, I close the open window of the file I just cropped and transferred. No other programs are running. This seems to be fairly standard stuff.
All was fine (although very slow) until two nights ago. When cropping, I received the error Could not complete your request because the scratch disks are full.
Once again, I researched scratch disk error messages, and I came to the conclusion I had no choice but to increase my RAM. So, I upgraded from 512MB to 2 GIG. (I successfully opened my PC and installed the RAM myself, confirming it on the Properties window. A major success, as the RAM was costly, and difficult to find, so I saved some money and time by installing it myself!)
With great enthusiasm and excitement, I opened the master Photoshop file again and the next image to be set to 300 ppi, cropped, and transferred over to a layer. But, when I tried to crop, I received the SAME sickening message, Could not complete your request because the scratch disks are full.
How can this be? I have four times the RAM I had before, which was costly, and I expected Photoshop to work swift and smooth with this new, more efficient increase in RAM. I am simply bewildered.
I tried changing my Scratch Disks to C: rather than Startup, but then I received the error message You currently have Adobe Photoshops primary Scratch and Windows primary paging file on the same volume, which can result in reduced performance. It is recommended that you set Adobe Photoshops primary Scratch volume to be on a different volume, preferably on a different physical drive.
More scratch disk torture.
None of this makes any sense, and, while Photoshop error messages are upsetting, they dont explain what you should do. I am not a programmer, and I am totally stumped.
I know that someone somewhere upgraded their RAM and still received a Scratch disk error too, so I hope someone can advise me on what I need to do next. I cannot continue my work and just spent a lot of money on RAM.
Thank you!

Answer 1

Hi, everyone: Thank you for your help with my problem. I appreciate everyones time and feedback, as well as the openness to help and explain (and further explain) what I do not fully understand. I learned a lot more, as Photoshop always introduces new things to learn and figure out. (One has to be part programmer to make PS work!) I appreciated the clear instructions and simple analogies too (for example, the head vs. suitcase was great).
Good news: The Scratch disk error has subsided for now!
I believe (although I am not certain) that the problem was something very simple after all, and I am embarassed. Only time will tell if this was really the culprit, for, if the Scratch disk error reappears later, it may be something else all together that is lurking in the background.
For now, it seems to have been a problem that Peter K.s advice helped me discover...
Peter's advice was to check the crop settings in the option bar for the common mistake of specifying pixels/cm rather than pixels/inch. My setting was correct (pixels/inch). However, I inadvertently used my pixel width number as inches in the Width setting, causing a MUCH bigger file than intended. When I changed the number, the Scratch disk error disappeared, putting me back in business! Thanks, Peter, for pointing me the direction that eventually revealed the problem.
Could it be this simple?
I kept working most of the night to test drive, free of the Scratch disk error for now! I hope the good fortune continues.
The lesson for others may be to check simple settings carefully first, before diving into the deep end of Photoshop.
Meanwhile, the new RAM is speeding up Photoshop considerably, so I feel like it was a good investment and will probably help safeguard against further Scratch disk errors, which I have had in the past. Also, Ive taken everyones advice to consider a dedicated, separate HD for Photoshop Scratch disk use. Ill plan that as a future/next investment. However, technical notes say an external HD isnt a good idea, but I dont have capacity for an internal HD, so it will have to be an external one. I would appreciate any words of wisdom here. Also, I still have to research SATA-II drives too (which are new to me) and ascertain if I can utilize one on my system. It may simply be too old for this technology, if it is new?
If interested, I also tried several things:
(1) Checking the space available on my HD again The pie chart indicated it was approximately half full (there was 46% free space, or 34.38 GB free space). I'll look for more to remove, but it seems reasonable to have 46 percent available, I think.
(2) Checking the need to DEFRAG The Analyzer said a DEFRAG was not needed, and, indeed, the color chart that plotted the files showed only a thin red line or two (red indicates fragmentation).
(3) Searching for more TEMP files to delete. I found 750 MB in TEMP files to delete, so, while not a lot, there were some still there. Previously I followed these directions to safely delete temporary Internet files: Ctrl Panel > Internet Options > General > Delete Files > Delete all offline content. But, when I navigated to C:\Windows\Temp, sure enough, I found more. Directly opening C:\Windows\Temp seems to be more effective. (I have done it that way in the past too, but lately I was trying to follow the "safe" way I had stumbled across recently.)
(4) Checking my Photoshop Memory Allocation, which was already set at 85% (which may be too high, so I may back it down later, but I did not change it for now). I remembered adjusting it some time ago, but I had forgotten by how much!
I also read the links that everyone recommended, as well as numerous other documents I found across the Web. Some introduced still more to learn/understand, of course.
Thank you once again. Ill resurrect or repost if the Scratch disk error haunts me again later, but, for now, a happy ending.