External drives randomly not loading correctly

Hello. On My late 2013 iMac I have three external drives connected, where one is used as my data drive (and I am one of the users who relocated my Home library folder to this data drive). The other external drives store TM backups and dmg images created by utilizing Disk Utility. I'm not technical, so much of my observations and assessments are based on repetitious results. After 14 months of flawless performance with this configuration I'm now suddenly experiencing two issues where--upon startup--one of the external drives often does not appear in Finder AND upon startup to Recovery when attempting to create a disk image I receive a "Restore Failure, not enough space on xxx to restore" error. On the drive loading issue this behavior occurs on each drive at varying times. On few occasions, all of them will load. Here is what I assume to be pertinent detail:

• When this began one week ago I was running 10.10.3; now I'm running 10.10.4
• All drives are formatted as Mac OS Extended (Journaled)
• On the drive that doesn't appear in Finder when I look at it in DU it shows read only and shows as Mac OS Extended--and does not show (Journaled)
• My initial suspicion was this behavior began after upgrading Adobe CC 2014 to the newly-released 2015 version; initially, when I reverted to a backup preceding that upgrade (and also reverted the Home library) I was able to create disk images and experienced several successful reboots with all drives loading but eventually this problem recurred. Eventually, I reverted to an "out-of-box backup, began installing several apps, and experienced several successful reboots and could also create disk images. But when I installed Adobe CC 2015 these two problems reappeared
• When booting to safe mode I've had numerous successful boots in terms of all drives showing (and no failed attempts). But I don't know what to do with that information
• Utilizing DU I repaired disks and permissions, where applicable, with no change
• In my Home directory I've deleted cashes sub folders, then moved on to deleting saved application state folders, both with no change
• I've disconnected unnecessary USB devices, including all but the external data drive but, still, that single drive will not load sometimes
• I've reset NVRAM and SMC
• I've moved these drives across varying USB ports, and reformatted them, and even loaded a couple drive's contents' to other (even brand new) drives

Do you have any suggestions? Thank you.