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Typically the bottleneck of an imaging operation is the write speed of the destination, not the read speed of the source, in which case the size of the image file written to the destination is more important than the amount of source data being backed up — unless CPU overhead of compression and encryption becomes a bottleneck, but that seems unlikely for a modern system.
As to the results, note that Reflect is showing megaBITS per second, while CrystalDiskMark shows megaBYTES. The conversion is to divide the former by 8 to get the latter. So your Reflect and CrystalDiskMark benchmarks aren’t TOO far apart (HDD results can be skewed depending on whether the read test was conducted on data located at the outer or inner area of the platters), but they’re a bit low for USB 3.0. I have a portable USB 3.0 drive that contains a 2.5” drive and I can get a pretty consistent 100 MB/s rate, give or take. Still, even taking the lowest speed shown in Reflect, 387 GB written at approximately 60 MB/s should take about 1 hour 45 minutes, not 5 hours.
Any chance the source drive may be having problems? If certain areas of the source drive incur high error rates due to surface defects or just a failing drive, that can drastically reduce imaging speed even if only for a portion of the overall operation.