Slow Sync Speed (25 MB/s) on MacBook Pro M1 between SSD and HDD RAID

[imraychien]

Thank you for developing such excellent software—it's been incredibly useful. I am currently experiencing the following issue:

Device: MacBook Pro M1
Source Drive: SanDisk Extreme Pro 2TB SSD (APFS Volume)
Backup Drives: Two Seagate 8TB Enterprise HDDs in RAID 1 (APFS Volume)

I am using two-way sync mode to back up approximately 2TB of data. The sustained transfer speed remains around 25 MB/s, and the process takes about 5 hours to complete.

During the first 4 hours, the processing rate stays at roughly 0.7 items/second. In the final hour, it gradually increases to about 3.5 items/second and processes over 200 items. However, the actual copy speed does not improve.

I’ve tried using other SSDs as the source drive but encountered the same behavior.
Is there any way to improve the sync speed?

2025-08-30 19.23.56.png (85.71 KiB) Viewed 156 times

[xCSxXenon]

Is it any faster transferring manually?
If not, it could be the cable, the port(s), how the drive(s) are connected, slow HDDs

[imraychien]

When transferring manually, the speed is normal for the hard drive. Is it because multithreading was not enabled in the settings? How should multithreading be configured?

[imraychien]

When transferring manually, the speed is normal for the hard drive. Is it because multithreading was not enabled in the settings? How should multithreading be configured?

Is it any faster transferring manually?
If not, it could be the cable, the port(s), how the drive(s) are connected, slow HDDs

[xCSxXenon]

Were you transferring the same data manually, or just random test data?
Make sure to try transferring the same data. File I/O overhead can be considerable, especially when dealing with large numbers of small files. RAID also impacts this, as there is more overhead for accessing them.
Threads is very unlikely to have any positive impact in this scenario.