Drive G: intermittently disappears

Drive U needs to be marked Inactive, only the C drive should be active.

Drive U and C should be swapped on their SATA connectors so C shows as Disk 0.

The drive you will have you Docs and personal data on should be placed in the Disk 1 position.

You can use this tut, under Tips, to include the data on the old OS drive to be recognized by the new install.
User Folders - Change Default Location

Taking the inputs from TVeblen and Britton30 above, I would think we need to check and restore some order in the way the internal drives get their drive letter and drive number.

1. As suggested by TVeblen, it should be ensured that his SSD and SSD alone is booting. To this end the OP should disconnect all other drives and confirm he is able to boot.

OK once this is established:

OP's internal drives ( He seems to have as many as seven internal HDDs -apart from the SSD. Am I right?) are getting drive letters haphazardly and I would think some order has to be restored. This can be set right only if he does a drive clean up removing all non-present devices and drive letters associated with them.

With all other drives - except the System drive SSD - disconnected, he should perform a drive clean up.

Download v 0.8.1 of the drivecleanup.zip from Drive Tools for Windows.

Unzip it to a folder, say drivecleanup.

You will have two folders Win32 and x64 each containing DriveCleanup.exe for 32 bit and 64 bit respectively.

Remove all USB storage devices from your system (except your Keyboard and mouse) and reboot.

Right click on the DriveCleanup.exe and run as administrator. (Use the *.exe file appropriate for your bit version of Windows.).

After the clean up act, reboot.

This will release all drive letters and make them available sequentially when he reconnects the internal drives like D, E, F, G, H etc., in that order and leave the higher alphabets to the numerous external drives which the Op seems to be connecting and disconnecting considering that he may have as many as eight USB ports on his comp.:)

As regards what Disk no. should disks , his SSD and the other internal drives, should get and how to organise it, I do remember that Britton30 brought out this problem how to get C drive to take on as Disk 0 in an old thread and so I would leave it to him. I forget in what way it was resolved. I read that thread just about 10 days back.

I know we are straying away from the main problem but would think we will be better equipped to deal with it when things get organised as above.

In particular I would like the OP to have only the system drive plus the problem drive only connected as an internal drive and tell us whether it still disappears on and off.

Perhaps this organisation itself may resolve the problem.

Yep, optical drives will take drive letters after C.

The OP after doing the drive cleanup will see his optical drive as D.

He can get into Disk Management change the drive letter to Z. That will make D available for his internal HDDs. Then he can manually assign drive letters to the internal drives as he may wish them to appear. Once this is done these will always stick with drive cleanup removing only the locked up drive letters associated with the now not-present devices.

(My virtual CDROM drive and two optical drives are assigned X,Y, Z and those always remain as X,Y,Z leaving the lower alphabets to the Internal and external HDDs.( Drive cleanup will only remove drive letters,and registry entries associated with non-present devices left over by those devices and release them) Drive letters assigned to the internal drives will always stick leaving the other alphabets available to the external drives.

Just to show how neatly they appear ( When I run drive cleanup under this condition drive letters I to W will be released if these are locked up in the registry and pointing to now not-present devices. )