Issue copying blk.dat files

Question

I've been running my node from a 500 GB external SSD but have run out of space to store the blockchain. I am copying files from the SSD to a larger USB 3 thumb drive using the command

rsync -ah --progress source/ destination/

The files are all the same size (134 MB) and type (Bitcoin blk.dat files). Some are copying over at the expected rate (~100 - 400 MB/s) however most are copying over extremely slow (~500 KB/s).

Fast:

Bitcoin/blocks/blk00013.dat
        133.96M 100%  181.47MB/s    0:00:00 (xfr#24, ir-chk=8221/8250)
Bitcoin/blocks/blk00014.dat
        134.10M 100%  126.75MB/s    0:00:01 (xfr#25, ir-chk=8220/8250)
Bitcoin/blocks/blk00015.dat
        134.22M 100%  401.25MB/s    0:00:00 (xfr#26, ir-chk=8219/8250)

Slow:

Bitcoin/blocks/blk00098.dat
        134.16M 100%  489.36kB/s    0:04:27 (xfr#109, ir-chk=8136/8250)
Bitcoin/blocks/blk00099.dat
        133.97M 100%  484.38kB/s    0:04:30 (xfr#110, ir-chk=8135/8250)
Bitcoin/blocks/blk00100.dat
        134.18M 100%  556.08kB/s    0:03:55 (xfr#111, ir-chk=8134/8250) 

             

At this rate it will take weeks to copy the whole blockchain over. I am wondering if this is something related to the blk.dat files or a hardware issue. I've stopped / started the process several times, using both cp and rsync

OS: Ubuntu 20.04.4 LTS (64-bit) GNOME version 3.36.8
Hardware: Dell XPS 15 9750
Source: 500 GB SSD external drive (connected via USB-C)
Destination: 2 TB thumb drive (connected via USB 3.0)

Any help appreciated.