zpool can not create a pool after using gdisk to prepare the device

I used gdisk(8) with a USB flash drive to:

1. zap (destroy) GPT data structures
2. blank out the MBR
3. (below) write a new GPT with a FreeBSD ZFS (A504) partition at /dev/da1p1

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root_at_mowa219-gjp4-8570p:~ # gdisk /dev/da1
GPT fdisk (gdisk) version 1.0.5

Partition table scan:
   MBR: not present
   BSD: not present
   APM: not present
   GPT: not present

Creating new GPT entries in memory.

Command (? for help): n
Partition number (1-128, default 1):
First sector (34-7827358, default = 2048) or {+-}size{KMGTP}:
Last sector (2048-7827358, default = 7827358) or {+-}size{KMGTP}:
Current type is A503 (FreeBSD UFS)
Hex code or GUID (L to show codes, Enter = A503): A504
Changed type of partition to 'FreeBSD ZFS'

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/da1.
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
The operation has completed successfully.
root_at_mowa219-gjp4-8570p:~ #

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I exported the pool that used the device at /dev/da0 (preparing for a 
disruptive test), removed both devices then reconnected the USB flash 
drive.

zpool can not create a pool, the file system is reportedly read-only. 
Please, why is this?

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root_at_mowa219-gjp4-8570p:~ # tail -n 0 -f /var/log/messages
Jan 12 06:44:44 mowa219-gjp4-8570p kernel: ugen0.6: <Kingston 
DataTraveler G2> at usbus0 (disconnected)
Jan 12 06:44:44 mowa219-gjp4-8570p kernel: umass0: at uhub1, port 3, 
addr 14 (disconnected)
Jan 12 06:44:44 mowa219-gjp4-8570p kernel: da0 at umass-sim0 bus 0 
scbus6 target 0 lun 0
Jan 12 06:44:44 mowa219-gjp4-8570p kernel: da0: <Kingston DataTraveler 
G2 1.00>  s/n 001D0F0CAABFF97115A00A15 detached
Jan 12 06:44:44 mowa219-gjp4-8570p kernel: (da0:umass-sim0:0:0:0): 
Periph destroyed
Jan 12 06:44:44 mowa219-gjp4-8570p kernel: umass0: detached
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: ugen0.6: <Kingston 
DataTraveler G2> at usbus0
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: umass0 on uhub1
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: umass0: <Kingston 
DataTraveler G2, class 0/0, rev 2.00/1.00, addr 15> on usbus0
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: umass0:  SCSI over Bulk-Only; 
quirks = 0xc100
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: umass0:6:0: Attached to scbus6
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: da0 at umass-sim0 bus 0 
scbus6 target 0 lun 0
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: da0: <Kingston DataTraveler 
G2 1.00> Removable Direct Access SCSI-2 device
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: da0: Serial Number 
001D0F0CAABFF97115A00A15
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: da0: 40.000MB/s transfers
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: da0: 3821MB (7827392 512 byte 
sectors)
Jan 12 06:44:48 mowa219-gjp4-8570p kernel: da0: quirks=0x2<NO_6_BYTE>
^C
root_at_mowa219-gjp4-8570p:~ # lsblk da0
DEVICE         MAJ:MIN SIZE TYPE LABEL MOUNT
da0              1:247 3.7G GPT - -
   <FREE>         -:-   1.0M -                                     - -
   da0p1          1:248 3.7G freebsd-zfs gpt/efiboot0 <ZFS>
root_at_mowa219-gjp4-8570p:~ # zpool create -m /media/sorry sorry /dev/da0p1
cannot open '/dev/da0p1': Read-only file system
root_at_mowa219-gjp4-8570p:~ #