Ah, thank you for taking an interest. I'm glad you agree it is odd!
I'm using a 2008 iMac running OSX 10.6.8 (Snow Leopard). IN all cases the drives were formatted with Apple's "Disk Utility" and backup gets done with "Super Duper" software, which can clone and then incrementally backup volumes.
Up until about a year ago, I have been using the "Lite" EZ-dock. THen I "upgraded" to the "Flex" dock, when I could no longer find the Lite docks. I needed more docs, because I had gotten another iMac (that one running OSX Yosemite). I like to back up all my computers.
My first problem was with a couple of 1TB drives (from either Seagate or WEstern DIgital... I'm not in office right now) that had originally been formatted with the Lite Dock two years ago (under Snow Leopard). I retired them after upgrading the backup volumes, now cloning onto 2TB drives. Around that time I got the FLEX dock and had been using it on the iMac with these newer drive mechanisms. (The Lite dock went to the other, newer "yosemite" iMac computer that gets used less often... I know, that sounds backwards, but the newer computer got the older dock, and I use it less often)
Very recently a new project came along with huge storage demands, that I wanted to backup onto those older 1TB drives that I'd put away last year. I put them into the FLEX dock, and they would backup for a while using SuperDuper, but the backup kept failing because of some error, something like "resource not found." WIth the backup thus corrupted, I had to try again with a full "erase, then copy" type of backup, actually a fresh clone, where the entire volume is written fresh from start to finish. This also failed, so then I suspected the drive mechanism had gone bad in storage.
I repeated all this with my second 1TB drive of the same age (I always keep two clones/backups, rotating one of them offsite). The second 1TB drive had the same problems, and at that point I doubted that both drives would have gone bad the same way. THat's when I retrieved the older "Lite" EZ-Dock, and tried all this again using that dock, and I had no problems, making two new clones.
So I decided to keep the older Lite dock on the iMac, because now I have lost faith in the newer Flex dock. Then I wanted to do my usual system backup (on the newer 2TB drives, formatted and backed up this year on the Flex dock), but when I tried to mount the drives on the Lite dock, they would not even read! The OS gave the message, this disk unreadable.
But if I hooked up the Flex dock, then the 2TB drives read and backup just fine.
I can give more details about the drive mechanisms, and give logs from the superduper backups, when I'm in office tomorrow.
thanks very much for your reply!