DD Rescue isn't really an imaging program as such, it does a straight
byte for byte copy from one HD to another - so the destination drive
has to be as large or larger than the source one even if there's only
a small part of the original drive in use. Imaging programs will take
the used space on a drive only, and also compress this so that you can
take an image of a 500Gb drive that's only got 60Gb used onto an 80Gb
drive - ddrescue would not be able to do that.

Clone Zilla is a free imaging program that has good reviews though I
didn't get on well with it a number of years ago. It's probably a lot
different now but so far haven't tried it out.

As for the USB to SATA adapter. If it works fine on lower capacity
drives then it *could* be power drain - have you tested it on a HD
that you know used to work fine in it ?