Transferring (stopping, resuming) file using rsync - macos

Question

I have an external hard-drive that I suspect is on its way out. At the minute, I can transfer files from it, but only for a while. Unfortunately, I have one single file that's >50GB in size. My solution to this is to use rsync to transfer this one particular file a bit at a time, leave the drive to rest (switch it off), and resume a little while later.
I'm using rsync --partial --progress --inplace --append -a /Volumes/Backup\ Drive/chris/Desktop/Recording\ Sessions/S1/Session\ 1/untitled ~/Desktop/temp to transfer it. (The file is in the untitled folder, which I'm moving into the temp folder) However, after having stopped it and resumed it, it seems to be over-writing the previous attempt at the file, meaning I don't really get any further.
Is there something I'm missing? :X
Thankyou ^_^
EDIT: Still don't know :\

Answer 1

Well, since this is a programming site, here's a program to do it. I tested it on OS X, but you should definitely test it on some small files first to make sure it does what you want:
#!/usr/bin/env python
import os
import sys
source = sys.argv[1]
target = sys.argv[2]
begin = int(sys.argv[3])
end = int(sys.argv[4])
mode = 'r+b' if os.path.exists(target) else 'w+b'
with open(source, 'rb') as source_file, open(target, mode) as target_file:
source_file.seek(begin)
target_file.seek(begin)
buffer = source_file.read(end - begin)
target_file.write(buffer)
You run this with four arguments: the source file, the destination, and two numbers. The first number is the byte count to start copying from (so on the first run you'd use 0). The second number is the byte count to copy until (not including). So on subsequent runs you'd always use the previous fourth argument as the new third argument (new begin equals old end). And just go on like that until it's done, using whatever sizes you like along the way.

Answer 2

I know this is related to macOS, but the best way to get all the files off a dying drive is with GNU ddrescue. I have no idea if this runs nicely on macOS, but you can always use a Linux live-usb to do this. You'll want to open a terminal and be either root (preferred) or use sudo.
Firstly, find the disk that you want to backup. This can be done by running the following. Make note of the partition name or disk name that you want to back up. Hard drives/flash drives will typically use the format sdX, where X is the drive letter. Partitions will be listed under sdX1, sdX2... etc. NVMe drives/partitions follow a similar naming convention.
lsblk -o name,size,label,fstype,model
Mount and change directory (cd) to a writable location that is bigger than the drive/partition you want to back up.
Now we are going to do a first pass over the drive/partition. This will do a first pass, without stopping on problematic sections. This will ensure that ddrescue does not cause any more damage by trying to access a bad section. Think of it like a hole in a sweater -- you wouldn't want to keep picking at the hole or it would get bigger. Run the following, with sdX replaced with the drive/partition name from earlier:
ddrescue -d /dev/sdX backup.img backup.logfile
the -d flag uses direct disk access and ignores the kernel cache, and the logfile is important in case the drive gets disconnected or the process stops somehow.
Run ddrescue again with the -r flag. This will retry bad sections 3 times. Feel free to run this a few times, but note that ddrescue cannot restore everything. From my experience it usually restores in the high 90%s, and many of the files are system files (aka not your personal files).
ddrescue -d -r3 /dev/sdX backup.img backup.logfile
Finally, you can use the image however you want. You can either mount it to copy the files off or use it in a virtual machine/burn it to a working drive with dd. Do note that the latter options will not always work if system critical files were damaged.
Good luck and remember to make backups!