The first question always has to be - are you sure it works with your system? S3 sleep is never guaranteed, requires compatible motherboard, software, and all devices, and it sounds like at least one of your USB controllers does not support it correctly. You can try fiddling with the USB config in the BIOS, and you can try plugging your flash drive into a different controller, e.g front vs back, or an internal plug, etc. Did it ever work for you before?
Here's the List from Tools / System Devices:
PCI Devices
00:00.0 Host bridge: Intel Corporation Sky Lake Host Bridge/DRAM Registers (rev 07)
00:02.0 VGA compatible controller: Intel Corporation Device 1902 (rev 06)
00:08.0 System peripheral: Intel Corporation Sky Lake Gaussian Mixture Model
00:14.0 USB controller: Intel Corporation Sunrise Point-H USB 3.0 xHCI Controller (rev 31)
00:14.2 Signal processing controller: Intel Corporation Sunrise Point-H Thermal subsystem (rev 31)
00:15.0 Signal processing controller: Intel Corporation Sunrise Point-H LPSS I2C Controller #0 (rev 31)
00:15.1 Signal processing controller: Intel Corporation Sunrise Point-H LPSS I2C Controller #1 (rev 31)
00:16.0 Communication controller: Intel Corporation Sunrise Point-H CSME HECI #1 (rev 31)
00:17.0 SATA controller: Intel Corporation Device a102 (rev 31)
00:1f.0 ISA bridge: Intel Corporation Sunrise Point-H LPC Controller (rev 31)
00:1f.2 Memory controller: Intel Corporation Sunrise Point-H PMC (rev 31)
00:1f.4 SMBus: Intel Corporation Sunrise Point-H SMBus (rev 31)
00:1f.6 Ethernet controller: Intel Corporation Ethernet Connection (2) I219-V (rev 31)
IOMMU Groups
/sys/kernel/iommu_groups/0/devices/0000:00:00.0
/sys/kernel/iommu_groups/1/devices/0000:00:02.0
/sys/kernel/iommu_groups/2/devices/0000:00:08.0
/sys/kernel/iommu_groups/3/devices/0000:00:14.0
/sys/kernel/iommu_groups/3/devices/0000:00:14.2
/sys/kernel/iommu_groups/4/devices/0000:00:15.0
/sys/kernel/iommu_groups/4/devices/0000:00:15.1
/sys/kernel/iommu_groups/5/devices/0000:00:16.0
/sys/kernel/iommu_groups/6/devices/0000:00:17.0
/sys/kernel/iommu_groups/7/devices/0000:00:1f.0
/sys/kernel/iommu_groups/7/devices/0000:00:1f.2
/sys/kernel/iommu_groups/7/devices/0000:00:1f.4
/sys/kernel/iommu_groups/7/devices/0000:00:1f.6
USB Devices
Bus 002 Device 002: ID 0781:5583 SanDisk Corp.
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
SCSI Devices
[0:0:0:0] disk SanDisk Ultra Fit 1.00 /dev/sda <- this disappears after wakeup, and comes up new (see below)
[1:0:0:0] disk ATA ADATA SP550 0C /dev/sdb
[2:0:0:0] disk ATA WDC WD30EFRX-68E 0A82 /dev/sdc
[3:0:0:0] disk ATA WDC WD30EFRX-68E 0A82 /dev/sdd
[4:0:0:0] disk ATA ST3000DM001-9YN1 CC4C /dev/sde
[5:0:0:0] disk ATA ST3000DM001-1CH1 CC44 /dev/sdf
[6:0:0:0] disk SanDisk Ultra Fit 1.00 /dev/sdg <- this disappears again after second wakeup and comes up new (see below)
[7:0:0:0] disk SanDisk Ultra Fit 1.00 /dev/sdh
That keeps going after every wakeup...
I found this line in the syslog :
kernel: ACPI: (supports S0 S3 S5)
I'm new to unRAID, switched from openmediavault. There worked everything fine, but the system was installed on a ssd not a usb thumb drive.
Maybe a little script, like searching for the usb drive after wakeup and mount it again, would help ?