3.x - OK
4.x - Not working

UF2 Bootloader v2.0.0-adafruit.9 SFHWRO
Model: Feather M0 Express
Board-ID: SAMD21G18A-Feather-M0-Express-v0

User: root

Initial USB connection dmesg:
[ 212.866387] usb 6-1: new full-speed USB device number 2 using ohci-platform
[ 213.058283] usb 6-1: New USB device found, idVendor=239a, idProduct=8023
[ 213.058317] usb 6-1: New USB device strings: Mfr=2, Product=3, SerialNumber=1
[ 213.058338] usb 6-1: Product: Feather M0 Express
[ 213.058358] usb 6-1: Manufacturer: Adafruit Industries LLC
[ 213.058378] usb 6-1: SerialNumber: 80C840FA0535E405A413E213D01150FF
[ 213.101387] usb 6-1: ctrl urb status -62 received
[ 213.106331] input: Adafruit Industries LLC Feather M0 Express as /devices/platform/fe3a0000.usb/usb6/6-1/6-1:1.3/0003:239A:8023.0006/input/input6
[ 213.164897] hid-generic 0003:239A:8023.0006: input,hidraw5: USB HID v1.11 Keyboard [Adafruit Industries LLC Feather M0 Express] on usb-fe3a0000.usb-1/input3
[ 213.168839] usb 6-1: USB disconnect, device number 2
[ 213.230462] usbcore: registered new interface driver cdc_acm
[ 213.230477] cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters
[ 213.249278] usbcore: registered new interface driver usb-storage
[ 213.257665] usbcore: registered new interface driver uas
[ 214.503344] usb 6-1: new full-speed USB device number 3 using ohci-platform
[ 214.694671] usb 6-1: New USB device found, idVendor=239a, idProduct=8023
[ 214.694706] usb 6-1: New USB device strings: Mfr=2, Product=3, SerialNumber=1
[ 214.694727] usb 6-1: Product: Feather M0 Express
[ 214.694748] usb 6-1: Manufacturer: Adafruit Industries LLC
[ 214.694768] usb 6-1: SerialNumber: 80C840FA0535E405A413E213D01150FF
[ 214.700275] cdc_acm 6-1:1.0: ttyACM0: USB ACM device
[ 214.706065] usb-storage 6-1:1.2: USB Mass Storage device detected
[ 214.708020] scsi host0: usb-storage 6-1:1.2
[ 224.729735] hid-generic 0003:239A:8023.0007: timeout initializing reports
[ 224.732838] input: Adafruit Industries LLC Feather M0 Express as /devices/platform/fe3a0000.usb/usb6/6-1/6-1:1.3/0003:239A:8023.0007/input/input7
[ 224.791960] hid-generic 0003:239A:8023.0007: input,hidraw5: USB HID v1.11 Keyboard [Adafruit Industries LLC Feather M0 Express] on usb-fe3a0000.usb-1/input3
[ 247.282391] usb 6-1: reset full-speed USB device number 3 using ohci-platform
[ 252.440373] usb 6-1: device descriptor read/64, error -110
[ 267.697900] usb 6-1: device descriptor read/64, error -110
[ 267.956923] usb 6-1: reset full-speed USB device number 3 using ohci-platform
[ 273.115081] usb 6-1: device descriptor read/64, error -110
[ 288.374190] usb 6-1: device descriptor read/64, error -110
[ 288.632093] usb 6-1: reset full-speed USB device number 3 using ohci-platform
[ 299.035480] usb 6-1: device not accepting address 3, error -110
[ 299.193315] usb 6-1: reset full-speed USB device number 3 using ohci-platform

Bootloader dmesg:
[ 1614.625615] usb 6-1: new full-speed USB device number 8 using ohci-platform
[ 1614.820757] usb 6-1: New USB device found, idVendor=239a, idProduct=001b
[ 1614.820791] usb 6-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 1614.820813] usb 6-1: Product: Feather M0 Express
[ 1614.820833] usb 6-1: Manufacturer: Adafruit Industries
[ 1614.820853] usb 6-1: SerialNumber: AF048C08504E5350312E314AFF05110D
[ 1614.823339] cdc_acm 6-1:1.0: ttyACM0: USB ACM device
[ 1614.826938] usb-storage 6-1:1.2: USB Mass Storage device detected
[ 1614.828425] scsi host1: usb-storage 6-1:1.2
[ 1614.838833] hid-generic 0003:239A:001B.0008: hiddev0,hidraw5: USB HID v1.00 Device [Adafruit Industries Feather M0 Express] on usb-fe3a0000.usb-1/input3
[ 1615.836969] scsi 1:0:0:0: Direct-Access Feather M0 Express 1.00 PQ: 0 ANSI: 2
[ 1615.848840] sd 1:0:0:0: [sda] 16000 512-byte logical blocks: (8.19 MB/7.81 MiB)
[ 1615.854808] sd 1:0:0:0: [sda] Write Protect is off
[ 1615.854848] sd 1:0:0:0: [sda] Mode Sense: 0f 00 00 00
[ 1615.860803] sd 1:0:0:0: [sda] No Caching mode page found
[ 1615.861378] sd 1:0:0:0: [sda] Assuming drive cache: write through
[ 1615.902830] sda:
[ 1615.943968] sd 1:0:0:0: [sda] Attached SCSI removable disk
[ 1684.757806] FAT-fs (sda): utf8 is not a recommended IO charset for FAT filesystems, filesystem will be case sensitive!