You must have had existing partion information from the previous attempt. Some times it may be easier to start fresh. If possible
dd if=/dev/zero of=/dev/sda
will zero or do an erase of the drive. Of course, this is assuming that you have no data on the device you want to retain. :)
If System Rescue CD is bringing up the device as read only, something tells me that your partitions may be a little confused or you have a hardware issues.
There is also the possibility that another error has occurred and the root partition is being mounted read-only. I can't say for sure as I'm having a hard time reading through all the issues you cite.
Not sure if you're aware of this...
Drive Mfgr's report size in SI (1000) units. GParted will report size in non-SI (1024) units.
Doing the math...((1000/1024)^3 x 750) = 698.4 GiB sounds about right in this case.
NOTE: increase exponent of (1000/1024) for each SI unit - k, M, G => 3