How to link more than one external HDD to be recognized as one HDD

Question

for the past 10 years or so i have been unintentionally collecting external HDD. some of them are 2T others are 250GB. is there a way to merge all of the drives so they are recognized as one single drive?

Answer 1

It is inevitable that someone will suggest using one of your extra drives to make a clone of your startup disk. This is a useful thing to have since it makes "regressing" to a former state of that drive possible, but it is important to understand how a clone differs from an archival backup strategy.
Basically, clones are functionally identical copies of volumes. (Drives may be partitioned into more than one volume, so each would have to be cloned individually to another volume to clone them all.) As generally & most practically implemented, clones are in effect snapshots of the state of a volume at some point in time. This means that any problems in the cloned volume will also be present in its clone, plus any problems that might result from an imperfect cloning process.
Thus, it is important that the clone is tested thoroughly before relying on it. If a clone of a boot volume, this means making sure that it will boot the computer & its system behaves normally as well as that it is free of any file system problems. This can take quite a bit of time -- it is easy to get complacent after a few clones have proved to be reliable & skip the testing, but if you are relying on it for backup this is not wise.
Also note that unless the cloning software has some provision for retaining old & deleted file versions -- and you use it -- each time you clone the volume you will lose those items, since the clone is intended to be a copy of the state of the drive at the time of cloning. So for example, if you overwrite an important file with another of the same name & file path by accident, you cannot retrieve the original one. Even if the software has a provision to retain the changes, it is not likely to do so in a way that makes it as easy to find or manage those file versions as with dedicated archival backup software.
Arguably, the best software for archiving is Leopard's Time Machine, with its spiffy user interface that makes it easy to see the state of the volume at any backed up previous time. And like most other archival backup software, it allows you to pick & choose what not to back up, & can backup files from multiple volumes onto a single one.
This type of software does not create a bootable volume like a clone does, so having both is a pretty good idea, especially when updating or upgrading the OS.