There are, however, things to be aware of when creating archives in this fashion. You must name your drive carefully and uniquely. Do not just call all your backup drives 'backup', because that risks user error when archives are plugged in again. (For instance, if you do a quick manual backup to the wrong drive, thereby overwriting an entire archive.) Use the final backup date and/or OS as the drive's name, perhaps; also consider putting a sticker on the drive itself, on which you add these important details.
Additionally, avoid the temptation to keep these drives attached for any length of time. They should be used to quickly find an old file or briefly work on something that's not possible on your current set-up. They're not there as a means to constantly access old data. (If you're constantly accessing certain files, that suggests they should be on your Mac's internal drive, or a separate 'current' data drive always accessible to it.)
Finally, do check archive drives are working every now and again. At the very least, spin them up every year or so. If the drive no longer appears to be working well, consider replacing it and copying the data across if the archived data remains important to you.
In event of disaster
As noted earlier, there are potential issues regarding back-ups. Drives over time can become unstable and unresponsive; and if you're not paying attention during a tired evening working with multiple drives, you might accidentally overwrite a drive you'd intended to never delete. In either case, it's imperative you immediately stop using the drive. The more a damaged drive is used, the worse it's likely to get. And, obviously, if you suddenly realise you're erroneously performing a clone to an archive drive, thereby overwriting data that's already there, stop the copy immediately.
There is no magic wand at either point. Chances are, you will lose some data and have to put in some work. Fortunately, software can often lend a helping hand. There are many products on the market for repairing and recovering Mac data, including: DiskWarrior; Data Rescue; FileSalvage; Disk Drill; and Data Recovery. The majority of these products provide a free, limited download, and then expect you to stump up the readies once they've proved their worth; and they all work in broadly the same way.
If you haven't overwritten your disk, but it's going a bit screwy, these apps will attempt to repair errors and directory structures. If you're extremely fortunate, this might be enough to salvage your data, although note that the process of combing through the drive can take many hours. If you're fortunate enough to find your drive springs back to life, immediately copy its data to another location. Do not expect it to continue working properly, even if it suddenly feels like new and is, for example, no longer making worrying clicking and grinding noises entirely at random.
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