External hard drive disconnects & does not reconnect upon restarting iMac

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New user to Macs. I have two external hd - My passport for backups (connects no issue) & I have tried two others for my Family Tree storage. One older black box and a new Seagate. Neither wants to reconnect. Have tried restarting, total reboot and no go. They seem to "go to sleep" after inactivity. My son-in-law somehow has made them reconnect but then they are shortly gone again. Would like to be able to have them reconnect/display without having to call him over every 2 day. Thanks!
 
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Your Mac's Specs
2019 iMac 27"; 2020 M1 MacBook Air; macOS up-to-date... always.
Which exact model drives are they? I'm not a fan of Seagate in general, and their external drives are sometimes not very Mac friendly. Also need your exact model/year Mac and version of OS X.
 
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Hi, Here is the info you requested.

- iMac (27-INCH, LATE 2013)
- OS X Yosemite Version 10.10.2

Seagate Backup Plus Slim Portable

I had the same issue with an older non-Seagate external hard drive

Thanks for your assistance....
 
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late 2012 mini w/SSD
Does the drive have its own power supply?
Go to the Disk Utility - does the drive show up there - can you mount (or Open) it there?
 
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The Seagate does not have its own power supply. The old black box did.
No, the Seagate does not show up in Disk Utility.

Any other ideas?

Thanks!
 
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It does show up when I restart but then goes to sleep after inactivity. Where can I change it so it doesn't fall asleep?

Thanks!
 
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Your Mac's Specs
2011 27" iMac, 1TB(partitioned) SSD, 20GB, OS X 10.11.6 El Capitan
A lot of the external drives, and it seems more so with a some of the later ones have a builtin sleep mode, but I'm not sure it can be changed. Check the manual or the manufacturer's site for support or info.

But normally unmounting, if required, and disconnecting and re-connecting the drive should get it to mount, unless Yosemite does something different.

If nothing else, the OP could try one of the Mac "Keep Awake" utilities to keep the drive alive like Keep Drive Spinning or one of the alternatives etc. and I think Insomnia??sp is another:
http://www.macupdate.com/app/mac/31158/keep-drive-spinning
 
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After reading the thread it sounds like it does not resolve my issue. My harddrive is actually dismounting (not showing in Finder). I have change the settings in the Energy saver but still happening.....
 
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Your Mac's Specs
2011 27" iMac, 1TB(partitioned) SSD, 20GB, OS X 10.11.6 El Capitan
Well, assuming that the drives, cables and connections are all working properly, have you tried one of the Mac "Keep Awake" utilities to keep the drive alive as I suggested previously.

And is the dive amounting wile you're using it or after sleep etc.?

It seems the problem has been around for a while according to just one long discussion:
https://discussions.apple.com/message/27358180?tstart=0#27358180
 

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